For example, as disclosed in JP-A-2015-60756 and JP-A-2016-34202, an electric power steering system mounted on a vehicle such as an automobile includes a load drive device in which an electric motor for assisting a steering operation of a steering wheel and a controller for controlling the electric motor are integrated with each other. The electric motor-integrated load drive device includes a board on which an electric circuit for driving and controlling the electric motor and an electronic component are mounted, an input/output connector through which an external device inputs or outputs power or a signal to or from the board, and a housing which accommodates the electric motor and the board.
The electric motor includes a coil, a motor terminal for supplying a current to the coil, and a rotor rotated by supplying the current to the coil. According to the load drive device disclosed in JP-A-2015-60756 and JP-A-2016-34202, the electric motor is incorporated in the housing so that the motor terminal and a shaft of the rotor protrude in an axial direction of a rotation axis of the electric motor. A frame (case) is connected to the housing, and the board is fixed to the frame. A cover is attached to the frame so as to cover the board.
According to JP-A-2015-60756, the load drive device includes a first board having a drive circuit mounted for driving the electric motor by supplying the current to the coil and a second board having a control circuit mounted for controlling driving of the electric motor. The first board and the second board are electrically connected to each other by an external connection terminal such as a bus bar. One end portion of the motor terminal of the electric motor is connected to the coil, and the other end portion of the motor terminal is electrically connected to the board by penetrating the frame.
Incidentally, various electronic devices are mounted on the vehicle in addition to a driving system such as an engine. In order to achieve weight reduction and miniaturization of the vehicle, the respective electronic devices also need miniaturization. The electric motor and the controller are integrated with each other in the electric power steering system. Accordingly, an occupied space of the electric power steering system inside the vehicle is reduced to a certain extent. However, even in a case where the integrated device is mounted on the vehicle, the vehicle needs further miniaturization. The controller includes a heat-generating electronic component having a high calorific value, such as a semiconductor switching element. Heat generated by the heat-generating electronic component causes a possibility that a malfunction or a failure may occur in the controller. Therefore, a countermeasure has been needed to radiate the heat generated by the heat-generating electronic component.
Therefore, according to JP-A-2015-60756 and JP-A-2016-34202, the frame (case) is connected to the housing on one side in the axial direction of the electric motor, and the board is fixed to the metal frame and the cover so that the board is perpendicular to the axial direction of the electric motor. The board is formed so as to have a size approximately the same as a diameter of the electric motor. In this manner, the electronic component configuring the electric circuit for driving and controlling the electric motor is mounted on a region where the board and the electric motor overlap with each other in the axial direction. The input/output connector whose outer shape is larger than that of other electronic components is mounted at a position where the board and the electric motor do not overlap with each other in the axial direction. According to JP-A-2016-34202, only one board is provided, and a drive circuit and a control circuit for the electric motor are mounted on the board. Furthermore, a heat conduction rate is improved as follows. The frame is used to serve as a heat sink, and an insulating heat radiation material is disposed between the electronic component mounted on a rear surface of the board and the frame.